Molecular Orbitals Flashcards
what are molecular otbitals?
molecular orbitals are formed when atomic orbitals combine, and they can hold a maximum of 2 electrons
what happens when two atomic orbitals combine?
they form two molecular orbitals:
bonding molecular orbital
anti-bonding molecular orbital
number of atomic orbitals combine = number of molecular orbitals made
difference between a bonding and an anti-bonding molecular orbital
bonding - lower energy, embraces both nuclei
anti-bonding - high energy, does not embrace both nuclei
what determines the strength of bonding between atoms?
the attraction between positively charged nuclei and negatively charged electrons in the bonding molecular orbital
how do bonding molecular orbitals differ due to polarity?
nonpolar - symmetrical
polar - asymmetrical
ionic - extreme asymmetry
what is electronegativity and its trends?
a measure of an atom’s attraction for bonding electrons
increases across a period and decreases down a group
what is a sigma bond?
a covalent bond formed by end-on overlap of atomic orbitals along the bond axis
what is a pi bond?
a covalent bond formed by side-on overlap of parallel atomic orbitals, perpendicular to the bond axis
how do sigma and pi bonds compare in strength?
sigma bonds are stronger due to greater orbital overlap
pi bonds are weaker due to less effective overlap
what is hybridisation?
the process of mixing atomic orbitals within an atom to form hybrid orbitals that are degenerate
how does carbon hybridise in alkanes?
carbon undergoes sp3 hybridisation
what is sp3 hybridisation?
one 2s orbital can mix with the three 2p orbitals to form four degenerate sp3 hybrid orbitals
how does carbon hybridise in alkenes?
carbon undergoes sp2 hybridisation
what is sp2 hybridisation?
one 2s orbital and two 2p orbitals hybridise to form three degenerate sp2 hybrid orbitals. the remaining 2p is left unhybridised
sp3 bonds and shape
sigma
tetrahedral
sp2 bonds and shape
1 sigma, 1 pi (due to unhybridized 2p orbital)
trigonal planar
how does carbon hybridise in alkynes?
carbon undergoes sp hybridization
what is sp hybridisation?
one 2s orbital and one 2p orbital hybridize to form two degenerate sp hybrid orbitals
sp bonds and shape
1 sigma, 2 pi (due to the two unhybridized 2p orbitals)
linear
how is benzene structured?
each carbon is sp2 hybridised forming sigma bonds. pi bonds are formed due to the unhybridized p orbitals.
the electrons are delocalised
what is a HOMO and a LUMO?
Highest Occupied Molecular Orbital - the highest-energy orbital containing electrons
Lowest Unoccupied Molecular Orbital - the lowest-energy empty orbital
how does molecular absorption affect colour?
electrons absorb energy to move from HOMO to LUMO
if energy absorbed is in the visible spectrum, the molecule appears coloured to the complementary colour of that absorbed
what is a chromophore?
a group of atoms responsible for absorbing visible light and giving colour to a compound
what is a conjugated system?
a system with alternating single and double bonds
